Radio remote-control aircraft system



Dec. 13, 1949 E. M. SORENSEN 2,490,844

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RADIO REMOTE-CONTROL AIRCRAFT SYSTEM Original Filed May 16, 1940 8 Sheets-Sheet 8 /00 IST Cyc- A E s Runas/z CYCLES E40/o @sca veg Patented Dec. 13, 1949 RADIO REMOTE-CONTROL AIRCRAFT SYSTEM Edward M. Sorensen, Dayton, Ohio; Helen S. Sorensen, administratrix. of said Edward M.

Sorensen, deceased Original application Ma 335,517, now Patent N ber` 8, 1946. March 24, 1941,

(Granted under the act oi.` amended april 30, 1928;

The invention described ,herein may be manufactured and used by or .for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to remote control systems and provides means for the control of a device or system locatedrenotelyfrom the source of control transmission, and is particularly described herein with reference to a construction for the control of a rotatable device, such as is adapted to be used in the controls o i `an airplane.

This application is a division of my application Serial No. 335,517 filed May 16, 1940 and entitled Radio remote control system, which has issued into Patent No. 2,408,819 dated October 8, 1946.

Remote control systems and apparatus are well known, but the devices heretofore used for this purpose have been subject to certain denite limitations. One class of remote control equipment, such as is commonly used in telemetering circuits, is satisfactory for the purpose intended and is capable of reproducing a continuously variable indication as made by a meter pointer, for example. However, this class of equipment is not useful for performing work, inasmuch as means have not been provided for operating a power means in accordance with the Signals reproduced. Another class of remote control equipment which has generally been used for operating power means at a remote point to correspond With the stimulus or stimuli applied at the control point comprises those systems commony known as step-by-step or stop-and-go systems, in which a plurality of predetermined settings are made and then by the signalling of impulses corresponding to one of the predetermined settings, the controlled apparatus is made to respond to the position signalled. This system constitutes the principle of the automatic dial telephone. However, it is subject to several defects. One of these is that positions intermediate the pre-set positions can not be obtained. This is particularly objectionable Where a continuous control of the controlled device is desired with very small increments of `motion being applied to the controls, as would be required for operation of a remotely controlledairplane. Another defect is that ii a large number of pre-set positions are attempted to be provided in order to.

reduce to a minimum the limitations of the previous defect, then the apparatus used becomes increasingly complicated, resulting in expensive equipment, the greater Y probability of failure due y 16, 1940, Serial N0. o. Divided and this. application` Serial No. 385,027

11 Claims.

2,408,819,- dated Octof-` March 3, 1883, as 370 0. G. 757) to the failure of any one of the larger number of elements used, and the increase in weight of the equipment. It will be obvious that for aircraft use, the weight of the equipment should be kept to a minimum. Still another class of remote control equipment depends for its operation upon the principle of the autosyn motor, but this type of equipment has the limitation that it cannot be used as a torque amplifier or, in other words, that no more power canbe obtained from the controlled device than What is applied to the control.

It is therefore. an object of the invention to provide a remote control which will give continuously variable operation of the controlled device, and will be capable of energizing or utilizing any amount of power at the controlled device, regardless of the energy applied to the control.

One of the diiiiculties in connection with the control of a `mechanical apparatus having a considerable amount of inertia is the tendency for the mechanical apparatus to continue to operate due to its inertia after the electrical control system has signalled the stopping point. This results in over-shooting or over-riding of the controlled device past the desired point of operation, resulting in hunting. It is an object of the invention to overcome or reduce this difficulty as much as practicable, and means have therei fore been provided for regulating the application of energy to the the distance the stopping point.

Another object of the invention is the provision of a remote control system suitable for the operation or" an airplane Without the aid of a human pilot in the craft.

Another object of the invention is the provision of means. with a remote control system, for causing the controlled device to automatically go to a pre-set position upon failure of the main control system. Such a provision is particularly useful in a remotely-controlled aircraft, where the pre-set position can be such as to cause the aircraft .to level off into a normal attitude of flight, or to turn the control of the aircraft over to some other system.

Other objects of the invention have to do with particular features and modications of the remote controlsystem, and will be more fully described hereinafter in connection with the draw'- ings.

The complete system comprises a variable frequency generating device Whose variation is continuousthroughout its range. Transmission of power means in inverse ratio to power means is from the desired said variable frequencies may be accomplished by any means known to the art, and as contemplated for purposes of the present invention, by the use of wire or radio transmission.

Reception is accomplished by the use of means common to the art, such as receivers and/ or appropriate amplifiers, the output of Vwhich is connected to a frequency measuring unit which converts variable frequencies into voltages with a magnitude proportional tothe frequency converted and independent of the amplitude changes of the frequencies above a threshold value. rIvhe output of said frequency measuring unit is coupled to a circuit having characteristics such that it is balanced by a steady voltage output of said frequency measuring unit, but is adapted to be unbalanced by any change in the voltage output, to an extent in proportion to said change. Thus, a change in the frequency transmitted will cause a change in the output voltage of the frequency measuring unit, which change of voltage will upset the balanced voltage, necessitating mechanical movement to reestablish same. The said mechanical movement is accomplished by a circuit network responsive to changes in the balanced condition and arranged to control the energization, direction of motion, and extent of motion of a power means geared to a mechanical element for rebalancing the voltage of the circuit connected to the output of the frequency measuring unit.

The arrangement is such that a frequency F creates a definite voltage E with a given angular setting of mechanical motion represented by the angle delta (A) Any change of frequency F will result in a new voltage value E1, upsetting the previous balance and causing mechanical rotation which in turn will reestablish a new balance whose angularity of mechanical rotation will be delta sub-1 (A1), the direction of frequency change determining the direction of mechanical motion.

In this invention, as illustrated in the drawings, the mechanical motion is accomplished by employing the resultant change of voltage created by the change of frequency to operate a bridge or vacuum tube, to cause a motor controlled by relays, as more fully hereinafter described, to drive a mechanism such as potentiometer arms to rebalance the bridge and to compensate for the voltage change caused by the frequency change.

It is thus seen that a change in the frequency transmitted is converted in accordance with the invention to a mechanical motion proportional to the frequency change. Increase or decrease of the frequency transmitted determines the direction of mechanical motion. Thereafter, the mechanical motion can be utilized for any desired purpose by means of a power take off, and in the specific application shown, is adapted to be connected to aircraft controls by instrumentalities such as a pulley and cables or other suitable means.

The invention will now be described in detail with reference to the drawings forming part of the application, in which:

Figure l is a schematic drawing showing the basic elements of the invention;

Figure 2 is a diagrammatic drawing of a basic circuit in which a mirror galvanometer and photoelectric cell is used as the detector;

Figure 2B is a particular circuit for the frequency measuring'unit;

Figure 3 shows an, arrangement of the basic circuit with a proportionalizing control circuit and centering circuit included;

Figure 3a shows the proportionalizing circuit separately;

AFigure 3b shows a variation of the proportionalizing circuit;

Figure 4 shows a preferred circuit arrangement which is similar to Figure 3 but incorporates certain modifications and additional features including a modified rectier circuit in the frequency measuring circuit, an emission compensation circuit for the relay control elements, and the proportionalizing circuit shown in Figure 3b;

Figure 5 is a simplified schematic drawing of the emission compensation circuit employed in Figure 4;

Figure 6 is a modification of the invention having a circuit adapted to be balanced at any steady voltage output of the frequency meter;

Figure 'l is a modified circuit employing a gas triocle tube in the frequency measuring unit and a two-stage direct current amplifier for the detector;

Figure 8 is a modified circuit in which a Wien type bridge is used;

Figure 9 is another modification of the invention using relays which operate on a plate current differential;

Figure 10 is a modication of the invention wherein a variable condenser is used as the bal- Figure 1 Referring to the drawings, Figure l schematically shows the arrangement of the units of the controlled device as adapted for operation by radio signals. The output of the receiver 20 is connected to the frequency measuring unit 2l, with or without amplification. The frequency measuring unit is shown with a direct current voltage output coupled to a bridge 22 having a rheostat 23 as one arm thereof. Across the diagonals of the bridge is connected a detector unit 2d for detecting the extent to which the bridge is unbalanced, by changes in the output of the frequency measuring unit. The detector is of a type that is sensitive to the direction of current fiow across the diagonal of the bridge and adapted to energize the relay 25 in response to one direction of current flow resulting from an increase in frequency of the received signal, and relay 2E in response to current flow in the other direction resulting from a decrease in frequency of the received signal. Relay 25 is adapted to in turn energize winding 2 of a power means in the form of a reversible motor 28, as illustrated, causing the motor to operate in one direction; and relay 2S is adapted to energize the other winding 29 of the motor, causing the motor to rotate in the opposite direction. The motor is connected to a reduction gear 3i] which has a power take-olf device, such as the pulley 3i illustrated, and is also mechanically connected to the arm of the rheostat 23 in such a manner as to rotate the potentiometer in the direction necessary to reestablish the balance of the bridge.

In accordance with the principles of operation heretofore described, it will be understood that for each given frequency, the frequency measuringunit will have a given value of voltage outv put. It will further be seenthat the bridge 22` Figure 2 The circuit shown in Figure 2 closely corresponds to the schematic arrangement of Figure l, but shows more Vfully an electro-mechanical detector in the form of a mirror galvanometer, light source, and photo-electric cell arrangement.

The Wheatstone bridge 22a is adapted to be balanced at a predetermined voltage when a steady voltage output of the frequency measuring unit is applied thereto. If a change of frequency takes place, there is caused a change of voltage output in the frequency measuring unit which unbalances the bridge circuit, setting up a current in the diagonal circuit of the bridge which includes the mirror galvanometer 32 so as to Acause the mirror galvanometer position in one direction in response to an increase in frequency, or to shift its position in the opposite direction in response to a decrease in frequency. A light source 33 impinges upon the mirror and the refiection of the light from the mirror is arranged to actuate one or the other of the elements 34 and v35 of the photoelectric cell 35, depending upony the position of the mirror. The photoelectric cell element so energized causes a rcurrent to ow in the corresponding relay 3"! or 38 which closes the correspondingy contacts 3Q` or 40. If'relay 39 is energized, then the circuit is'closedto relay `25e, which operates and which is provided with a heavy enough armature to carry the current for the motor. Likewise, if relay 38 is energized, the circuitis closed to actuate the heavy duty relay 26a for closing thelcircuit tothe other: winding of the motor.

TheinventionY is not limited to any particular frequency measuring unit. One type is shown in Figure 22, merely by way of example. There is used in conjunction with thefrequency measuring unit, a rectier so as to provide a direct current output.

The frequency measuring circuit shown in Figure 2e, as well as the other frequency measuring circuits shown in following modifications, provide a voltage output which is proportional to frequency and not affected by amplitude above a threshold value. Their operation depends upon converting the applied alternating voltage to a substantially square top wave and by the use of a capacity 4| and resistor 42 as a frequency timing network. The output voltage of the frequency measuring unit is dependent upon the charge and discharge of said capacity, said charge and discharge being accomplished by the square top wave generating device. The frequency range that. can be measured will be limited by the value of capacity used, thereby necessitating different values of capacities for different upper frequency limits. `This limitation is determined by the length of time it takes the condenser 4I to acquire its Ycharge through a to shift its 'i given value of resistance 42, this time constant being a fixed value dependent upon the value of the capacity and the resistor. Thus, it can be readily seen that the length of time the alternating voltage is positive or negative is solely dependent upon its frequency, therebyautomatica-lly controlling the size of capacity that would be used in this circuit for a given maximum frequency. It will therefore be obvious that the capacitance of the condenser 4l should be in inverse relation to the maximum value of the range of frequency which is to be employed. Likewise, it will be seen that it is desirable to limit the range of frequency toas small an amount as is feasible in connection with the sensitivity that is desired for the controlled device. In this connection, it may be pointed out that quite satisfactory sensitivity of a control for aircraft use can be obtained by a frequency ratio of 1 to 1.8, that is, for a frequency range of 1GO toV 18o cycles, sensitivity of the rotatable member which is designed to be connected to the aircraft control member can be held within the reasonable accuracy required for this operation;

Figure 3 The invention shown in Figure 3 is Vsimilar to the basic circuit shown in Figure 2, but adds a centering circuit for the motor and a proportionalizing circuit for controlling the action of the relays 25h and 25h. This circuit also employs modifications in the frequency measuring unit, the balancing circuit connected to the output of the frequency measuring unit, and the detector. The motor unit is also shown in greater detail.

Referring to the drawing, it will be seen that the frequency measuring unit comprises a pentode tube 43 having the characteristics of very sharp cut-olf and high amplification factor. The plate current of this tube iiattens off or saturates at a definite value of grid excitation. The operation of this form of frequency measuring unit is as follows: The incoming signal is fedV to the grid 44 of an amplifying tube 45 being, as illustrated, one-half of a twin triode tube. The two resistors 45 and 41 prevent the grid 44 from going excessively positive. The 'output of tube 45 is applied to grid 48 of tube 43 and drives the plate current of said tube to a saturated value on the positive peaks, thereby generating a substantially fiat top wave in its plate circuit 49.

Condenser Mb and resistors 5U and 5| constitute a frequency timing network and function similarly to the corresponding elements in Figure 2a so far as the matter of acquiring a charge of condenser tlib is concerned. The voltage discharged from condenser 4 lb is applied to the grid of amplifying tube 52 which, as illustrated, constitutes the other half of the twin triode tube, of which tube 45 is a part. The voltage impulses present in the primary of transformer 53 connected to the plate 54 are isolated above ground and are rectified by twin diode tube 55 and filtered by the pi network filter 56 to provide a direct current output negative at terminal 51 and positive at terminal 58. A portion of this direct current voltage is placed across a potentiometer 59. The remainder of this voltage is distributed over the circuit comprising the arm 60 of the potentiometer, resistors 6| and B2 and back to the negative side 5'1' of the filter. The negative side of the filter is also connected to the negative pole of a battery 63 and the potentiometerarm 60 is also connected to the positivepole of a battery 64. The potentiometer, the potentiometer arm circuit, and the batteries 63 and 64 constitute in this modification of the control, the balancng circuit which is adapted to be balanced by a steady output voltage of the frequency measuring unit and to be unbalanced by a change in voltage out put of the frequency measuring unit. The arm 60 of the potentiometer is mechanically connected to the reduction gear 30b so as to be adjusted to rebalance the circuit in response to operation of the motor 28h.

The detector in this modication is a twin triode tube 10 having two grids 1l and 12 independent of each other, two plates 13 and 14, and two cathodes 15 and 16. The grid 1l of this tube is connected in series with battery 63 at its positive pole. The other grid 12 is connected in series with the second battery 64 to its negative pole. The potentials of the batteries are dependent upon the available output voltage of the frequency measuring unit and the sensitivity required of the controlled device. By way of illustration, the potential of the batteries as used in one model of the control and as considered in the operation hereinafter described is L11/2 volts for battery 63 and 101/2 volts for battery 64. The plates 13 and 14 are connected to relays 26b and 25h respectively, which relays control the direction of rotation and the energizations of the motor 28h.

In explaining the operation of the abovementioned circuit, the following assumptions and conditions will be set forth: Assume, for example, that with a potential of 25 volts across the output of the rectiier tube 55 and filter 56, that the potentiometer 59 has its arm 60 set to give a potential of l volts measured from the arm to the negative side 51 of the rectifier output. Assume further that the tube has a plate current of equal value in each plate of the order of 1 milliamp. Assume further that the relays 25b and 26b located in each of the plate circuits of tube 19 will remain closed with a current of 2 millamps. Assume that the grid potential for the plate current given will be -3 volts. This condition will eXist as long as the potential of 25 volts exists across the output 51 and 59 of the rectiiier and filter unit 56, and as long as the potentiometer arm 60 remains in its position so as to provide a potential of volts between its arm 69 and the negative side 51 of the rectier output. Now, if a signal of lower frequency is applied to the grid 44 of tube 45, a lower potential will exist across the output 51 and 58 of the filter 56. This will produce a lower potential than 15 volts as measured from the arm 66 of the potentiometer to the negative side 51 of the rectified output. When this lower potential exists, it causes a less negative potential to exist on the grid 1| of tube 10, thus causing an increase of plate current in the plate 13 of said tube. It will be noted that under normal conditions with the grid potentials at -3 volts, relays 25h and 261 are open. When relay 26b is closed, a voltage flows in the field of motor 28h. The motor is connected so that this will cause same to rotate in the direction to secure a potential which will satisfy the grid voltage for a potential of -3 volts on each grid, and vice versa for relay h, relay 25lo being controlled by an increase of potential on grid 12. The normal grid potential is created by bias cells 63 and 64 in series with each grid. The static potential of these cells is different in the fact that one potential going to grid 1| has its plus side connected to the grid, and its negative side connected to the grid return. Bias cell 64 has its negative side connected to grid 12, with its positive side connected to the grid return and to the potentiometer circuit. Connecting across resistors 6| and 62 is a denite potential which in this case is l5 volts. This causes a potential of -3 Volts to exist on grid 1l to grid return point 11, and on grid 12 to grid return point 11. In the event a potential change from 15 volts occurs across resistors 6l and 62, it will cause a lesser negative voltage to be present on either grid 1l or 12, depending on which way the potential changes; i. e., if the voltage is less than 15 volts, grid 1I will have a less negative potential applied to it, grid 12 having a more negative potential applied to it. If the potential is greater than 15 volts, grid 1I will have a more negative potential applied to it; grid 12 having a less negative potential applied to it. In the event the potential on the grid becomes less negative from the pre-set point, this will in turn cause an increase in plate current, relays 25b and 25h being so adjusted that a given increase in plate current will cause the arm to close, completing a circuit to cause rotation of the motor.

The motor unit shown in Figure 3 is similar to the motor units of Figures 1 and 2, but is shown in greater detail, as including a magnetic brake and clutch 89. When the motor is energized, one of coils 8l or 62 has a potential across it which creates a magnetic field which is designed to engage the motor to the reduction gear 36h. When the potential no longer exists, the magnetic eld collapses in the magnetic brake and clutch, disengaging motor 28h and being further designed to brake the inertia of reduction gear 30h. In this manner, the motor is prevented from hunting and over-ride to a great extent. Limit switches operated by cams 83 and 84 are provided to limit the angular rotation of the device to maintain the operation of the motor within the angular limits of the potentiometer or other balancing element.

CENTERING CIRCUIT In the event that there is a failure in plate supply voltage, filament supply voltage, transmission link or any interruption of the frequency which is set up on grid 4d of triode section 45, a centering circuit 85 will go into operation, causing 4the control or power take-off Sib to move to a pre-set position. This operation is accomplished by capacitatively coupling through condenser 86 the potential of alternating voltage present on plate 81 of tube 45 to bridge rectifier 88, the output thereof being connected to relay 89 whose arm 99 is closed to contact 9i as long as an alternating voltage of suflcient amplitude exists on grid 44. While arm 99 is making contact with contact 9 i, relays 25D and 26b have complete control of the power take-oil 3lb. In the event of a failure, arm moves back to contact 92, which connects through contact 93 with contact 94 or 95, contact 94 controlling the operation of the motor in one direction, and contact 95 controlling the operation in the other direction. Contacts 94 and 95 are controlled by cam 96 mounted on the reduction gear 39h. A cam lift 91 constitutes the centering or pre-set position of the controlled device which is to be obtained in the event of failure as above specified. When the cam is rotated counter-clockwise, as shown, the lift 91 will lift the contact 93 to a point midway between contacts 94 and 95, at which time rWhen vneon tube Vthe circuit to the motor will be open, causing deenergization of same. Likewise, if the contact 93 were resting on the cam arc of greater diameter, clockwise rotation of the cam would cause the contact 93 to disengage contact 95 as it dropped down the cam lift 91, thereby causing rotatiton of the controlled device to the same pre-set position, at which point the motor circuit is-opened.

This circuit can also be used to transfer control for the controlled device to some other control means, such as a set of `gym-controlled instrun ments as used in an automatic pilot, or to any other iixed control means. It will be understood that by connecting in series with relay 8S other relays or switch means which are associated with various elements of the control circuit so as to be closed under normal operating conditions, a fail ure of any suchelenients will cause the relay associated therewith to open, with its arm up thecontact for the centering circuit, causing operation of Asaine in the manner above described. Likewise, it will be understood that a single relay circuit, such as shown and described in Figure 3, may be associated with any particular element of the control, so that upon failure of the same,

the-centering circuit will go into operation.

PROPORTIONALIZING CIRCUIT To secure greater sensitivity and provide a higher degree of accuracy in the actuation or potentiometer arm G0, is is necessary to make further ,provision for controlling the inertia ci the motor andthe controlled device, so as to prem vent huntingand over-riding and to insure substantially deadbeat stopping action. rlhis is accomplished by means of the LJroportionaliaing circuits shown in Figure 3, associated with the plate controlled circuits of relays Zlb and 223D. Each .proportionalizing circuit shown functions alike in response to an increasein the plate current ofplates lf3 or lil, and therefore a descripn tion of .the one will suiiice for both.

The circuit is shown separately in Figure 3e, if.- lustrated with a tricde half of .the twin triode 10. The circuit will be seen to'consist ofa condenser le@ and a neon tube lill connected .in series and parallel to the relay 25h, condenser |00 being connected to the plate side ofthe relay. At the junction of the neon tube I! andcondenser |00 is a resistor m2, the other .terminal of resistor' |92 being connected as at i3 tothe plate supply potential. To the junction of the ineen tube |i3| and the relay winding is connected a resistor it@ having its other tern minal running to side |63 of the so ce of potential. Grid l2 of vacuum tube has .nally impressed thereon the negative V3 Volt potential described above in connection with the function of vacuum tube as the detector. The opposite side of the grid potential as at its to cathode le, also connected to the negative side of the plate potential |06.

To illustrate the action oi the above circuit, assume that there Vis an increase in plate rent in the plate le in the Vorder -ainperes due to a less negative potent in response to an increased Voltage the fi ncy in asuring due to in frequency of thereceived signal. This causes condenser to take an increased charge tircugh resistor |32 Vat the time a potential isset up across resistor i of cient .value to cause ignition of neon tube itl, 46| ignites, a discharge path making tube corresponding to one through relay 25b and neon tube |0| is provided for condenser l 00. The current iiow through relay 25b caused by the discharge of condenser 00 is in excess oi the current required to close the relay, so that the same will close during the discharge of the condenser. Thus, the motor is put in operation for a brief interval of time. When condenser lili? is discharged, the neon tube extinguishes, since resistance |02 is provided with a great enough resistance to prevent the passing oi current of sunicient value to maintain the neon tube ignited. Condenser |00 being discharged and neon tube |ii| being extinguished, the circuit immediately starts to recharge condenser |00 through resistance EQ2, whereupon the cycle will be repeated. As the potential 'on grid 'l2 becomes still less negative, the plate current in plate "it will be steadily increasing so that the rate at which the neon tube |0| and condenser |00 operate to actuate relay 25b will increase. When the plate current is of suicient value to maintain the relay continuously closed, the proportionalizing circuit has no more effect until the grid potential is made more negative to a value slightly less than the potential at which the relay is continuously closed. Then the proportionalizing circuit will again actuate the relay intermittently at a decreasing rate until the grid potential reaches its normal value at which the potentiometer circuit is balanced, which in this case is negative 3 volts.

In this manner, energy will be supplied to the motor or other power means in a pulsating manner, with the rate of pulsations being in direct proportion to the distance the controlled device is from the point corresponding to the signal frequency. However, when the reduction gear is of a sizeable ratio, the kmotion of the controlled element will appear to be substantially continuous and progressive in its rate of increase or decrease.

The values of condenser |00 and resistance |02 determine the rapidity with which the proportionalizing circuit will operate for a given plate current. The value of resistor |04 determines the plate current required to cause operation oi' the relay in cooperation with the neon tube and condenser.

It will be seen that the proportionalizing circuit operates as a function of current, 'wherein as the plate current is increased across the proportionalizing circuit to an amountcorresponding to the ionization potential of the neon tube, the circuit will start to function, due to the increased voltage drop across resistor |04. As the voltage continues to increase beyond the limits of the proportionalizing circuit, the proportionalizing control will stop and the device will move at its iull rate, but as soon as the mechanical elements of the control approach the desired setting, the proportionalizing circuit functions at a vrate equal to the amount the control is out of balance. Thus, as the control is brought into balance, the proportional rate becomes slower until and within a very close limit. The proportional time is cut down until the balance is just reached and difficulties from .hunting or over-shooting are avoided. In this sense, the circuit may be described as an anticipator circuit.

`In .Figure 3b is shown a variation of the proportionalizing circuit in which a condenser |01 is connected in parallel with resistance |02. This arrangement has the effect of increasing the intervals Vol time required for the condenser |00 to receive its charge, and similarly, the time to disi charge.

v through condenser Y 1i Consequently, the circuit will operate withY less rapidity and the relay 25b will be closed and open for longer intervals of time.

It will be understood that by adjusting the values of resistor IM or the spring tension on proportionalizing circuit be maintained throughout the operation of the controlled device.

Figure 4 The circuit shown in Figure 4 is similar to that shown in Figure 3, except for the following described modifications: In the frequency measuring unit the amplifying tube'45 for the input signal of Figure 3 and the amplifying tube 52 for the at top wave voltage pulses produced by the tube 43 have been omitted. The transformer 53 and the filter 56 of Figure 3 have been replaced in Figure 4 by a more compact rectifier circuit which comprises twin diode rectifier tube I2 coupled directly to the timing condenser GIC. Condensers I 2| and |22 are connected in series arrangement between cathode terminal |23, and the anode terminal |24 of tube |20. These condensers serve as filter condensers to filter out any alternating voltage which might be present from cathode |23 to ground or plate |24 to ground,

and also serve to. provide a conducting path for the electrons during the charge of the condensers Since the one leg of the alternating current applied to this rectifier circuit dic is connected to ground |25, a return ground connection |26 is connected to the output of the rectifier circuit at the juncltion between the two condensers I2! and |22.

It will thus berseen that the output terminals |21 and |23 of the rectifier circuit will have substantially equal and opposite polarities with respect to the ground reference |26, and that the voltage directli7 across'these terminals is double the voltage of the alternating current which is applied to the rectifier circuit, so that in this manner the voltage supplied by the frequency measuring unit to the potentiometer has been rectied, amplified, and isolated above ground, thus obviating the necessity of the transformer 53 and amplifying tubes of Figure 3, thereby re-v sulting in considerable saving in weight and greater simplicity. This rectier circuit also has the advantage of providing more linearity of response; i. e., for a frequency change of 2 to 1, there results a voltage change of 2 to 1, whereas with the circuit as shown in Figure 3, it may be difficult to obtain linearity of response due to the characteristics of triode 52 and transformer In the balancing circuit connected to the output of the rectier at points |21 and |28 and which is adaptedV to be balanced at a steady Vvoltage output Yof the frequency measuring unit, a dual potentiometer I 3@ is used. Dual potentiometers Isli and resistors ISI and |32 serve as the load resistance for the rectifier' I 2Q'. The junction point |31 returns toY ground |38 through the emissio-ncompensation network hereinafter described, but may he omitted as this circuit does not require the load resistance to be grounded to provide a potential. The ground is used to assure a balance of the two potentials to the ground point, and is for the purpose of the radio control circuit. Batteries |39 and I 4I! have the same potential but are connected to the arms IM and |42 of the dual potentiometer in' opposite polarity arrangement so as to buck the potenw tial existing across the potentiometer arms and will provide a desired potential across points |513 and IM when the potential existing across the arms is at the desired relation to the potential of the batteries.

The advantage of a dual potentiometer in this circuit is that it provides an equal load on both sides of the rectiiier circuit between arm IM to ground and arm |42 to ground. Another reason for using dual potentiometers in this circuit is that it is difficult to obtain potentiometers of the Wire wound variety having a high enough resistance in a single unit. Thus, it is advantageous to use two potentiometers, thereby getting twice the resistance and providing a satisfactory load impedance for the rectifier.

'Iwin triode detector tube 1i)c has its grids connected to the points |43 and IM and functions in response to an unbalance of the potentiometer circuit to control the relay circuits to the motor in the same manner as the detector described in Figure 3.

It can be seen that in lieu of the detector tube 1B, a meter with a series resistance can be connected across the points |43 and |44. Then, when the potential existing across the arms IM and |42 of the potentiometer is equal, the potential or current across points |43 and |44 as read on the meter will be zero, but when the frequency is raised or decreased, the voltage output of the rectifier would increase or decrease respectively, causing a current to flow in the meter in a direction corresponding to the direction of voltage change. To nullify this current and bring the reading to zero, it would be necessary to move the potentiometer arms in a direction to bring about nulliiication of the current. By Calibrating the potentiometer in frequency, the frequency could be read directly on a dial. In this way, there is constructed a novel frequency meter having the advantages of being simple, economical of manufacture, and quick measurements.

The proportionalizing circuit used in Figure 4 has incorporated a condenser |91c in parallel with resistance H32c and functions in the same manner as the circuits described in connection with Figures 3a and 3b.

The motor circuit is the same and functions in the same manner as shown in Figure 3.

The centering circuit of Figure 4 functions in exactly the same manner as that shown in Figure 3, but is provided with an amplifier tube |50 for the power supplied to the relay 89".

Compensation for emission variation due to changes in filament potential is provided in this circuit, and is accomplished in the manner shown and next described in detail in connection with Figure 5. Tube |50 has a second plate I5I arranged in diode relation to the cathode, for use with the emission compensation circuit. There is also provided forthis circuit a resistance |52 and battery |53 arranged in parallel to the diode plate I 5| with respect to the grids of tube 10. Additional emission compensation is provided by connecting the cathode of tube 18 to a portion of the filament potential by means of a variable resistor or potentiometer |54..

EMISSION COMPENSATION CIRCUIT Figure 5 `circuit is to provide emission compensation 'to 'overcome .plate current variations due to increase or decreaseof filament potentials vfrom a normal value. In a direct current amplifier, such as twin triode detector tube 70 represents in the control, or in any vamplifier where agreat amount of stability is necessary for variations in supply potentials, this circuit will be found useful. It is a known fact that when the ilament temperature is increased, there are a greater number of elec'- trons emitting from said filament. This is also true with a cathode type Vacuum tube. The emission compensator depends for its operation on-the use of the Edison effect. The potential changes set up froma diode plate to its cathode will vary with the temperature of the cathode. The manner in which this potential varies is controlled at Vthe rate at which the cathode heats up or cools off, depending upon the supply potential to said cathode.

This circuit comprises a diode tube |69 having its plate Iii, corresponding to plate i! of tube A|5|l yin Figure 4, connected to the grid return |52 of -grid |63 corresponding to the grids the twindetector tube 'me of Figure 4. The grid return is also connected by a resistor |54 through `,battery |65 to the-cathode |66 of said diode tube. 'Resistor Miel and battery |65 correspond to re- -sistor `|52 and battery |53 of Figure 4. The battery and `cathode are commonly connected to ground, or as shown, to B- terminal |68. With this circuit, the bias on grid |53 will vary, by measuring the biasvirom grid |53 to the cathode `|559 -of triode tube Htl, in a direct relation with the filament potential as supplied to lament I1 l, assuming lament |l'l is supplied from the same potential which supplies the filament |72 of the diode. The manner in which this bias potential varies V:follows -very closely to the value of voltage needed to bias the grid to maintain the plate current of plate |12, as measured in meter |13, at the same value which was flowing prior to a change in filament potential.

.Battery |65 has a positive potential with re spect .to the cathode I t9. Thus it can be seen Ythat the diodeplate 15| will have a positive potential with respect to cathode t6. Any changes in `current being =drawn by said diode Will be rep resented by the voltage drop across resistor |64. Thus it .can be readily shown that for any Value of emission which takes place across the diodey a `corresponding voltage will be set up between points |52 and |61.

In some cases it has been found necessary to include some of the lament supply potential voltage `cases where the emission of the tube varies a great deal for a small filament potential change. Byplacing a potentiometer |14 (corresponding to .potentiometer |55 of Figure 3) in the return cirati'on across'said lament circuit will also be lrepresented -on the grid. The potentiometer should be adjusted to provide the desired amoun-t of bias voltage. Fixed resistances `may be added to each side ci the potentiometer if necessary. This agdd- 't 'ed bias voltage, together with the Voltage asset Yup across said diode |65, provides the necessary control to Yhold the plate current of said vtriode at a steady value for large changes in lament potential. These changes in iilament potential may be as great as 109%; that is to say, for 'a .tube -wliich has a normal operating voltageof 6.3 volts, the filament supply voltage may vary fr'om 4 `to 8 volts, with the emission remaining constant over this range.

as a bias voltage on the grid, especially in 14 It may also bepointed out thatthis additional `voltagermay be obtained from the plate supply voltage in those cases where the plate supply voltage is obtained from the same source as the filament vsupply voltage, as any variation in compensated, and responsive only to the signal applied-to the grids from the output of the rectifier circuit. This is very important where the 'filament `voltage can not be maintained constant, as would probably be the casein the power supply of aircraft. In the event of a change in filament potential variation without provision for emission compensation in response to such change relays c and 21T@ controlling the motor circuit would be affected by changes in the plate current of plates 13 and 14C, which would not be caused by a change in the grid signal, and would therefore result in erratic operation of the motor.

Figure 6 Figure 6 shows schematically the circuit of the controlled unit of a remote control system wherein a balancing circuit is used which is adapted to be balanced at any steady output Voltage of the frequency measuring unit. The basic elements of this circuit are as shown in Figure l.. The balancing circuit comprises a vbattery |8|l havinga potentiometer |85 connected in parallel with the battery. rThe detector 24d is arranged with respect to the terminals |83 and |84 of the .frequency measuring unit and With respect to the terminals |85 and |35 of the balancing circuit so as to be sensitive to a difference in potential across these two sets of terminals. Assume, for example, that a potential of 10 Volts exists Yacross terminals |83 and |84. Then the motor Will be quiescent when the potential across terminals |85 and |85 is adjusted to be ten volts. But until the Apotential is so adjusted, or if adjusted, and a change in potential across terminals |83 and |84 takes place, the detector will be sensitive to such deviation or change and will energize relay 25d or 26d, depending on the polarity of the difference in potential between the two sets of terminals, to cause one of the relays to energize .the motor. The arm of potentiometer |8| being connected -to the motor through reduction gear d, will be rotated to rebalance the potential across terminals .I and |85 to match that across |83 and |813. To illustrate this action further, assume that the frequency of the received signal increases. Then the voltage across terminals |83 and |84 will be, say, 12 Volts. This will ,Cvtof cathode '69 a portion of any Votagg Var create a differential between these terminals and terminals |85 a-nd |86 so that the detector will actuate the motor to rebalance the potentiometer |8-| toset up a potential of 12 volts across terminals |85 and |86, at which time, the relay will fall out and the motor will de-energize.

Figue 7 comprising resistors v|84 and |95 and Acondenser Aid which constitutes the frequency timing net- Vof said rectifier is connected to an amplifying twin triode 213| and again amplified by twin triode 292. The relays 2lie and 26e operate in the same manner as described in connection with Figure 3 to control the operation of motor 28E which rebalances potentiometer 203 located in the cathode circuit of twin triode tube 2li l.

Figure 8 The form of invention shown in Figure 8 Vincludes a frequency measuring circuit and otherwise conforms in general to the principles of the invention, except that a bridge circuit of the Wien type is further employed as a frequency measuring element. circuit provides a means of changing the audio voltage of a given frequency into a denite voltage which is independent of amplitude variation and solely dependent on frequency variation. In the tube circuit following the frequency meter the relay 265 hasV contacts located on the pull-in side and the drop-out side. Thus, if the relay is open, it makes up the motor circuit for righthand rotation. If the relay is in, it makes up the circuit for left-hand rotation. The motor is mechanically connected to a gain control circuit 26S of the tube following the frequency meter which is adapted to provide a fixed grid bias at any steady frequency regardless of the frequency amplitude. The motor is also connected to the arms of the Wien bridge. Assuming that the Wien bridge is balanced at a given frequency and the frequency is raised, the Wien bridge becomes unbalanced and has an output across its detector arms which trip the relay Ztl located in the plate circuit of the amplifier tube of the Wien bridge. This will close the power to the motor circuit. The motor then adjusts the gain control circuit to restore the grid bias to its fixed value and drives the Wien bridge in the direction of a nulL When a null is reached` the relay located in the plate circuit of the amplier tube of the Wien bridge drops out causing` the motor to cease operation. Thus it can be seen that the frequencies varying either high or low from a previously set point cause the motor to rotate in a desired direction to rebalance the bridge. The relay 295 controlling the direction of operation of the motor is always sensitive to a decrease or increase of the received signal because the plate current of tube Zilli is adjusted to the value at which the relay trips by means of the gain control circuit 266 with each new frequencyV value or setting of the controlled device.

Figure 9 In the form of invention shown in Figure 9, and referring to the circuit arrangement shown to the left of the dotted line B-B, part A, the output of the electronic frequency measuring device is not rectified but is left in its fiat top wave pulses, and its amplitude which is being fed to the following tube is controlled by the motor which drives the radio control; thus if the frequency increases, the voltage as measured from The frequency measuring t the arm of the potentiometer to the cathode circuit increases. This causes the motor to rotate in a given direction to turn the potentiometer arm to a lower value of potential, or to the same potential that was present on the grid of the tube following the frequency meter previous to the increase in frequency.

Reference is now made to the circuit arrangement embraced in that part of Figure 9 located to the right of dotted line B-B, generally denoted as part C. This part of the circuit changes the audio signal Voltage into angular or lineal mechanical motion proportional to frequency. The operation is outlined as follows: Twin tube 21B includes two triode sections 2H and 2|2. Section 2l l is so biased that it takes a lesser signal on grid 213 to operate to pull in the relay 25g in the plate circuit of said section 2| l, and it takes a greater signal on grid 2 I4 of section 2|2 to operate relay 26g located in the plate circuit of said section 212.

The functioning of the circuit is elaborated more fully as follows: Signal voltage e is present on the grid of tube 2l5. This signal voltage is suicient to cause relay 25g to be closed, which leaves the contacts open. Voltage e applied to the grid of tube 2l5 is not suicient to cause relay 2tlg in the plate circuit of section 212 to close. It is therefore seen that the circuit including the potentiometer 23, twin triode tube ZIB, and the relays, is balanced at voltage e and the motor is quiescent. If the signal becomes greater, relay 2f? will close causing operation of the motor 28g in one direction, as the contacts of this relay 26g are closed when the relay closes. The motor will continue to operate until it has adjusted the potentiometer, to which it is mechanically connected, to a lower value of potential to reestablish the signal voltage e on grid of tube 2l5 at which point relay 26g drops out and the motor becomes quiescent. If the signal becomes less on the grid of tube 215, the relay 25g in the plate circuit of section 2H drops out, closing the contact and operating the motor 28g in an opposite direction and adjusting the potentiometer to provide a higher Value of potential to reestablish the signal voltage e. Thus it can be seen that the relays 25g and 26g are arranged and controlled so as to cause the motor to be quiescent when a steady frequency is received, to be operated in one direction in response to an increase in the frequency of the signal received, and to be operated in the other direction in response to a decrease in the frequency of the signal received.

It will be understood that the same operation can be obtained by biasing grids 2I3 and 2M equally but adjusting relays 25g and 26g so that they close at different Values, relay 25 being closed at the normal grid signal bias and adapted to open in response to a decrease in such signal bias, and grid 26g being open at the normal signal bias and adapted to close in response to an increase in such bias.

Figure 10 Figure l0 depicts a circuit having basically the same elements as the circuit shown in Figure 4. This circuit differs from that shown in Figure 4 inasmuch as a voltage balancing device, such as the potentiometer, has been replaced by a variable capacity 22e, said variable capacity being attached to the reduction gear 3th attached to motor 28h. The motor is controlled by relays 25h and 26h through twin triode 70h which is controlled by the amount of voltage which exists be- `The proportionalizing circuitY shown in Figure 311 is also associated with' each relay. The value of voltage appearing betweenY points '|4311 and |4411 is in accordance with the frequency of the signal Voltage applied between points 22| and 222. Control of said voltage is accomplished by varying the value of condenser 220. Assuming the grids '|2h and 7|11 of twin triode '|011 are balanced at a negative l volt potential, then that condition makes relays 2511 and 2611 idle. For said negative l volt bias condition, assume that the potential between points |4311 and |4411 is Volts for some frequency being impressed at points'22| and 222. In the event there is a change in frequency between points 22| andf222, this will at once change the voltage between points 4311 and |44f1. lThis alsounbalances the voltage set up on grids '|211 and 11, thereby making one grid more negative and one grid less negative.v This causes the motor 2811 to operate in a predetermined direction, thereby causing condenser 220 mechanically connected to motor to rotate in the direction necessary to acquire a valueto reestablish the potential of twenty volts existing between points |4311 and |4411, at which time the voltage on grids '|211 and 'H11 returns to its normal value of negative one'volt, causing the relayV to open and the motor to stop.

The circuit of Figure 10 is otherwise similar to that of Figure 4 except that the centering circuit and emission compensation circuit are not shown.

As was shown in connection with Figure 4, this circuit lends itself for use as a novel frequency meter. By disconnecting that part of the circuit prior to points |4311 and |4411 from the grids of twin triode tube 1011 and attaching a meter in series with a resistance across said points, current flow or voltage across these terminals can be detected. Then, by Calibrating the variable condenser in frequency, and adjusting same until the reading of the meter is a predetermined value, the frequency of the input signal across points 22| and 222 can be read upon a dial. This circuit when used as a frequency meterhas as its advantages a high degree of accuracy and an exceedingly wide range of measurement as well as the advantages pointed out in connection with the meter of Figure 4.

MULTI-CONTROL SYSTEM FOR AIRCRAFT Figure 11 Figure 11 shows schematically theapparatus andv method of operation of same for a remotely controlled aircraft, in which control is provided for the throttles, rudders, ailerons, and elevators;

The ground station (which might readily be in'- stalled in another aircraft used as the control) has four control elements representing and corresponding to the throttle, rudder, ailerons, andv elevators. These control elements are mechanically connected to four oscillators having fre- 225, the output of the transformer being fedV toha modulator and amplifier of any well known' v The output of theampii'er is connected'to modulate a 'radio frequency transmitter and is transmittedbymeans of antennae 18 4 226. The transmission is exactly that of radio any distance, being limited only by the power of the transmitting station, and the characteristics of the transmitter. The signal is received at the antennae 22'! of the controlled aircraft and is applied to a radio receiver of ivellknown commercial design having good mera/tingy The output signal of the radio receiver audio oscillators; filter passes frequencies of to 19o cycles; a second nlter passes only frequencies of 250 to 5% cycles; a third lter passes only 7f3@ to 1400 cycles; and a The outputs of these niters are connected to a such as described in Figure 4 for operating the throttle, rudder, ailerons, and elevators respectively. The power circuit such as used in Figure 4 is shown one ofthe control elements spending movement of the corresponding aircraft control in the remotely controlled aircraft. The system requires only one radio transmitter for the the hydraulic power circuit of of Figure 4, Figure 4 is 2. In combination, an aircraft, a robot control mechanism in said aircraft, means for actuating said robot control mechanism from a remote frequencies, and chanical motion of said aircraft.

energized by i wave, means in said aircraft for receiving said signals, band pass filter means for separating said signals into the separable ranges of frequencies generated by each of said control elements, means responsive to each range for converting said modulation frequencies into mechanical motion proportional to the instantaneous value of said means for applying said meto the corresponding controls 3. The invention as defined in claim 1 in which -there is further provided means for automatically restoring the aircraft controls tothe normal neutral position,

including switching means associated with said motive power means adapted to restore the aircraft control to neutral and magnetic relay means adapted to maintain the sys- Vtem responsive to radio control when energized l.

by the received radio signal and to transfer the 'control to said switching means when deenergized.

4. The invention as defined in claim 1 in which vthere is further provided a second control systemV in said aircraft, and means operatively coupled to said mechanical motion means for positioning the airplane controls on a predetermined neutral position, and magnetic relay means continuously actuation of said robot control mechanism to said second control system upon a failure of said first mentioned control system.

5. A remote control system for aircraft comprising a radio transmitting antenna at a transf mitting station and a receiving antenna on said aircraft, a source of variable modulating frequency energy at the transmitting station and frequency responsive means at the receiving antenna on the aircraft for converting said modulating frequency energy into voltages proportional to the instantaneous modulating frequency and independent of the amplitude of said energy, said frequency responsive means comprising Vacuum tube means to limit the signal voltage, a condenser and resistance in the output circuit of said limiter, and means to average the voltage pulses appearing across said output resistor, means for translating said voltages into mechamcal motion and applying said motion to a control of said aircraft, said translating means comprising a reversible motor, magnetic switching means for controlling the motor and vacuum tube means excited by said averaged output voltage for actuating said switching means.

v6. A remote control system for aircraft comprising a transmitting antenna at the transmitting station and a receiving antenna on said aircraft, a source of wave enegy at the transmitting station, aircraft controls associated with said source at the transmitting station to vary the modulating frequency of said source in accordance with incremental settings of the control element, and means at the receiving antenna on the aircraft for converting said wave energy into voltages proportional to the instantaneous modulating frequency said converting means comprising vacuum tube means to limit the signal voltage, a condenser and a pair of reversely connected rectiers the received signal, for transferring l a control element simulating one of the y-in the output circuit ofv said limiter and the aircraft for translating said an outand means on voltages into mechanical motion for the purpose of controlling the movement in direction and elevation of the aircraft.

7. In an aircraft remote control system for effecting a plurality of continuously variable controls at a point removed from the aircraft, a source of radio wave energy, a control element simulating each of the controls on the aircraft, means associated With veach control element adapted to produce kmodulating frequencies varying in value with each setting of the control element, a receiver on the aircraft having a plurality of modulation frequency band pass filters, means for converting the modulation frequency signal from each band pass lter into voltages proportional to the modulating frequency, means for translating each of said voltages into mechanical motion of an aircraft control corresponding to a simulated control element at said remote point, saidV converting means comprising vacuum tube means to limit the signal voltage, a condenser associated with said voltage limiter, a pair of reversely connected rectiflers and an output resistor, said translating means comprising a reversible motor, magnetic switching means for controlling the motor, and vacuum tube means for actuating said switching means.

8. In a remote control system for aircraft having a plurality of movable control elements at the transmitting station and a plurality of controlled elements on the aircraft corresponding to said control elements respectively, each of said controlled elements on the aircraft being calibrated to correspond with its respective control element at the transmitting station and adapted continuously to respond to movements of said control elements, a source of radio wave energy at the transmitting station, means associated with each control element adapted to produce modulating frequencies varying in value to correspond with each incremental setting of the control element, means for transmitting said frequencies on a single radio carrier to said aircraft, means on the aircraft for receiving and separating said modulating frequencies into separate channels, and means for translating each of said channel frequencies into mechanical position of said aircraft control elements corresponding in direction and extent to the change in modulating frequency produced by each control element at said source, said translating means comprising vacuum tube means to limit the signal voltage, a condenser and a pair of reversely connected rectiflers in the output circuit of said limiter and a resistor in the output circuit of said rectiers, said mechanical positioning means comprising a reversible motor adapted to move the airplane controls, magnetic switching means for controlling the motor and vacuum tube means excited by the Voltage appearing across said output resistor for actuating said switching means.

9. In a control system for aircraft, a control source at a radio transmitting station and including a modulating frequency generating device, means for transmitting the frequencies generated to said aircraft, means on said aircraft for receiving said transmission frequencies, said means put resistance for said rectifiers,

including an electronic frequency circuit for translating said modulation frequencies into voltages proportional to the instantaneous frequencies said electronic circuit comprising vacuum tube means to limit the signal voltage, a condenser and a pair of reversely connected rectifiers in the output circuit of said limiter and an output resistor in the output circuit of said rectiers, and means for applying said output voltages to effect control of one of the aircraft control elements, said applying means comprising a reversible motor for positioning said aircraft control, mag netic switching means for controlling the motor, vacuum tube means for actuating said magnetic switching means, adjustable potentiometer contacts on said output resistor for varying the input excitation of said vacuum tube means, and a mechanical connection from the motor to said adjustable contacts for restoring equilibrium.

10. A remote control system for aircraft comprising a control source at the transmitting station including a movable control element means for generating radio Wave energy means for modulating said energy and varying continuously the modulating frequency in predetermined relation to movements of said element, means positioned on the aircraft and communicatively connected to said control source including a frequency measuring device for translating instantaneous frequencies of said source into proportional voltages, and means for applying said proportional voltages to a control system adapted to position a control element governing the motion of said aircraft, said frequency measuring device comprising a gas triode energized by the frequency to be translated, a condenser-resistor network connected to the catnode of said triode, an amplier, a bridge rectiiicr and an output resistor, said applying means, comprising a reversible motor connected to an aircraft control, magnetic relay means for controlling the motor and vacuum tube means excited from said output resistor for actuating said relay means.

11. In a remote control system for aircraft having a radio transmitting station in which there is means for modulating the transmitted radio frequency, and a movable control element simulating one of the aircraft controls adapted to Ivary the modulating frequency in accordance with the position of said control element, the improvement in the controlled portion comprising a radio receiver on the aircraft, means for converting the received instantaneous modua lating frequency into a voltage proportional to said frequency, and a positioning system responsive to said proportional voltage for positioning an aircraft control, said converting means comprising vacuum tube means to limit the signal 22 voltage, a condenser associated with said voltage limiter, an amplier and a full wave rectier, said positioning system comprising a reversible motor adapted to position one of said aircraft controls, a magnet switch energized from the output of said rectier for controlling the motor, a poten tiometer for biasing said amplier and a mechanical connection from the motor for positioning the potentiometer; frequency bridge means for measuring the modulating frequency and adapted to interrupt said motor circuit when the bridge is balanced, said bridge comprising a resistance capacity network with input and output terminals and having two mechanically adjustable resistance arms positioned by said reversible motor, a second amplier excited from the bridge output terminals, said bridge input being connected to the signal input, a second full wave rectier energized ,by said second ampliner output and a second magnetic switch energized from said second rectifier output with contacts in the motor supply circuit.

EDWARD M. SORENSEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,390,288 Hammond Sept, 13, 1921 1,597,416 Mirick Aug. 24, 1926 1,818,708 Hammond Aug. 11, 1931 1,867,171 Ranger July 12, 1932 1,924,857 Hodgman Aug. 29, 1933 1,941,615 Mirick Jan. 2, 1934 1,942,587 Whitman Jan. 9, 1934 1,958,258 Alexanderson May s, 1934 1,991,443 Becker Feb. 19, 1935 2,025,054 Hodgman Dec. 24, 1935 2,063,610 Linsell Dec. 8, 1936 2,077,401 Crosby Apr. 29, 1937 2,109,475 Fanning Mar. 1, 1938 2,152,336 Van Loon Mar. 28, 1939 2,153,782 Weber Apr. 11, 1939 2,165,800 Koch July 11, 1939 2,175,320 Runge et al Oct. 10, 1939 2,257,203 Thacker Sept. 30, 1941 2,397,475 Dinga Apr. 2, 1946 2,408,819 Sorensen Oct. 8, 1946 OTHER REFERENCES Electrical Engineering, for Jan. 1936, pages 42-45, (Copy in Sci. Lib.) 

